Fabrication method for phase change diode memory cells

ABSTRACT

A method of forming a diode structure for a phase-change data storage array, having multiple thin film layers adapted to form a plurality of data storage cell diodes is disclosed. The method includes depositing a first diode layer of CuInSe material on a substrate and depositing a second diode layer of phase-change material on the first diode layer.

This application is a divisional of U.S. Pat. No. 6,979,838, having U.S.patent application Ser. No. 10/654,189, filed Sep. 3, 2003.

FIELD OF THE INVENTION

The present invention relates to ultra-high density data storagedevices. More particularly, the present invention involves ultra-highdensity data storage devices having diode memory cells using materialscontaining copper, indium, and selenium (CIS), and methods offabricating the diode memory cells.

BACKGROUND OF THE INVENTION

Electronic devices, such as palm computers, digital cameras and cellulartelephones, are becoming more compact and miniature, even as theyincorporate more sophisticated data processing and storage circuitry.Moreover, types of digital communication other than text are becomingmuch more common, such as video, audio and graphics, requiring massiveamounts of data to convey the complex information inherent therein.These developments have created an enormous demand for new storagetechnologies that are capable of handling more complex data at a lowercost and in a much more compact package. Efforts are now underway toenable the storage of data on a scale of ten nanometers (100 angstroms)up to hundreds of nanometers, referred to herein as “ultra-high densitydata storage.”

One method of storing data at ultra-high densities involves utilizing adirected energy beam. As used herein a “directed energy beam” means abeam of particles, such as electrons, or a beam of photons or otherelectromagnetic energy, to heat the medium so that it changes states. Asused herein, “state” is defined broadly to include any type of physicalchange of a material, whether from one form to another, such ascrystalline to amorphous, or from one structure or phase to another,such as different crystalline structures. As used herein, the term“phase change” means a change between different states in a material.

A state change may be accomplished by changing a material fromcrystalline to amorphous, or the reverse, by the application of anelectron or light beam. To change from the amorphous to crystallinestate, beam power density is increased so as to locally heat the mediumto a crystallization temperature. The beam is left on long enough toallow the medium to anneal into its crystalline state. To change fromcrystalline to amorphous state, the beam power density is increased to alevel high enough to locally melt the medium and then rapidly decreasedso as to allow the medium to cool before it can reanneal. To read fromthe storage medium, a lower-energy beam is directed to the storage areato cause activity, such as current flow representative of the state ofthe storage area.

To effectively sense contrasts in states or phases of phase-changematerials, a diode may be formed having a junction for sensing carrierflow in response to an electron or light beam focused on a data storagememory cell in the phase-change layer. Such diode junctions are utilizedfor carrier detection in photovoltaic devices, in which light beamsimpact the diode, and in cathodovoltaic devices, in which electron beamsare directed to the diode. Photovoltaic devices include phototransistordevices and photodiode devices. Cathodovoltaic devices includecathodotransistor devices and cathododiode devices. In addition, diodejunctions may be utilized for carrier flow detection in photoluminescentand cathodoluminescent devices. Reference is made to copending patentapplication Ser. No. 10/286,010 [HP 20020-6667] filed on Oct. 31, 2002for a further description of the structure and function of diodejunctions in these devices.

Diode junction layers need to be composed of materials having electricalproperties suitable for generating a desired carrier flow across thediode junction. As used herein, the term “carrier flow” refers to eitherelectron current or the flow of holes, depending on whether thematerials are n-type or p-type. As used herein, the term “materials”includes all kinds and types of compounds, alloys and other combinationsof elements. Various types of junctions may be formed in the context ofthe above data storage devices, such as heterojunctions, homojunctions,and Schottky junctions, in order to achieve the desired detectionresults.

SUMMARY OF THE INVENTION

One embodiment comprises a method of forming a diode structure for aphase-change data storage array, having multiple thin film layersadapted to form a plurality of data storage cell diodes, wherein themethod comprises depositing a first diode layer of CIS material on asubstrate; and depositing a second diode layer of phase-change materialon the first diode layer.

Other aspects and advantages of the present invention will becomeapparent from the following detailed description, which, when taken inconjunction with the accompanying drawings, illustrates by way ofexample the principles of the invention.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a partial side view of an exemplary data storage system;

FIG. 2 is a plan view of the storage system shown in FIG. 1;

FIG. 3 is a schematic view of a portion of the storage system shown inFIG. 1;

FIG. 4 is a schematic side view of the diode structure according to oneembodiment of the present invention;

FIG. 5 is a flow chart describing a method for fabricating the diodestructure shown in FIG. 4;

FIG. 6 is a bar graph showing EBIC diode characteristics according tofour diode embodiments of the present invention; and

FIG. 7 is a graph showing the current-voltage characteristics of a diodeaccording to one of the embodiments of FIG. 6.

The same numerals in the Figures are assigned to similar elements in allthe figures. Embodiments of the invention are discussed below withreference to the Figures. However, those skilled in the art will readilyappreciate that the detailed description given herein with respect tothese figures is for explanatory purposes as the invention extendsbeyond these limited embodiments.

DETAILED DESCRIPTION

Reference will now be made to the exemplary embodiments illustrated inthe drawings, and specific language will be used herein to describe thesame. It will nevertheless be understood that no limitation of the scopeof the invention is thereby intended. Alterations and furthermodifications of the inventive features illustrated herein, andadditional applications of the principles of the inventions asillustrated herein, which would occur to one skilled in the relevant artand having possession of this disclosure, are to be considered withinthe scope of the invention.

FIG. 1 shows an exemplary ultra-high density data storage system 100having electron emitters 102 and 104 and a phase-change data storagemedium 106. Electron emitter 102 and 104 are mounted above the storagemedium 106, that has a number of storage areas, such as 108, that areimpacted by electron beams from the emitters. Micromovers, based onmicro-electromechanical systems (MEMS) technology, cause relativemovement between the phase-change medium 106 and the electron emitters102. Micromover 110 is connected to the storage medium 106 and moves itrelative to the emitters 102, 104, so that each emitter can impact anumber of different data storage areas.

FIG. 2 is a top view of the cross section A—A in FIG. 1, showing themicro-mover 110 fabricated to scan the medium 106 in the X and Ydirections. The storage medium 106 is supported two sets of thin-walledmicro-fabricated flexible beams or springs 112 and 114 which flex toallow the medium 106 to move in the X direction with respect to asupporting frame 122. A second set of springs 116 and 118 are connectedbetween the supporting frame 122 and the outer casing 120 and flex toallow the medium 106 and frame 122 to move in the Y direction withrespect to the casing 120. The field emitters scan over the medium, orthe medium scans over the field emitters in the X-Y directions byelectrostatic, electromagnetic or piezoelectric means known in the art.

FIG. 3 shows an exemplary diode structure 140 comprising thephase-change layer 106 described above and a second diode layer 142forming a diode junction 144 at the interface of the two layers.Typically, phase-change layer 106 and second diode layer 142 havedifferent electrical characteristics, to encourage the movement ofcarriers across the junction. For example, phase change layer 106 may ben-type and second diode layer 142 may be p-type. Doping may be used tochange or enhance the electrical characteristics of each layer. Anexternal circuit 146 is connected to the diode to impress a voltageacross the junction so as to create a bias, either in a reversedirection or in a forward direction, depending on the needs of thecircuit. An output 148 is generated by circuit 146 and is representativeof the magnitude of carrier flow across junction 144.

Reading or detecting is carried out by directing a narrow beam ofelectrons 105 onto the surface of phase change layer 106 at data storagearea 108. The incident electrons excite electron-hole pairs 107 near thesurface of the storage area 108. The diode 140 is reversed-biased byexternal circuit 146 so that the minority carriers that are generated bythe incident electrons drift toward the diode junction 144. Minoritycarriers that do not recombine with majority carriers before reachingthe junction are swept across the junction, causing a current to flow inthe external biasing circuit 146.

Writing onto diode 140 is accomplished by increasing the power densityof the electron beam 105 enough to locally alter some property of thediode at storage area 108, such as collection efficiency of minoritycarriers. In FIG. 4, a schematic side view of a diode configuration 200in accordance with one embodiment of the present invention is shown. Aphase-change layer 202 is composed of indium and selenium in an indiumselenide (InSe) compound which can be reversibly changed from anamorphous to a crystalline state by heating and cooling the InSecompound at suitable rates. This state change is useful for datastorage, since the amorphous state is less conductive that thecrystalline state. The InSe compound may be any InSe-type material. Asused herein the terms “InSe compound,” “InSe-type material” and “InSelayer” mean any compound of InSe, In₂Se₃ or a material in the grouphaving the formula In_(x)Se_(1−x).

When the InSe layer 202 is changed from the crystalline state to theamorphous state at the storage area 212, the electrical properties ofthe InSe layer significantly change. Accordingly, the number of carriersswept across the diode junction will be significantly different in theamorphous state than when InSe is in a crystalline state.

In FIG. 4, the second diode layer 204 is disposed below the phase-changelayer 202 to form a diode junction 206. Since polycrystalline InSe-typematerials are naturally n-type, the second diode layer 204 may be ap-type semiconductor layer or capable of being p-doped. The second diodelayer 204 may be made of a material of copper indium selenide (CuInSe),also known as CIS. The terms “CIS,” “CIS compound” or “CIS material”refer to a material having any ratio of copper, indium, and selenium,such as CuInSe₂, including but not limited to CIS doped with gallium(CIGS). The term “CIGS” refers to CuInSe doped with gallium to formvarious stoichiometric materials of copper indium gallium selenideincluding but not limited to (Cu(In, Ga)Se₂) and Cu(In_(1−x)Ga_(x))Se₂.

CIS materials are desirable for memory diode materials for a number ofreasons. CIS compounds are similar to InSe-type materials, differing inthe additional Cu element and a change in crystalline structure.Although the structures of these two compounds are different, the diodeinterface between layers of the two compounds is effective. Also, CIScan be readily doped with a p-type dopant which is required to work withInSe. Further, the CIS layer can be formed in the same vacuum system asan InSe layer, enabling the formation of a clean high quality interfacebetween the two layers. This diode structure also reduces processingcosts, since the time required for an additional fabrication stage iseliminated. In addition, CIS and CIGS materials tend to have relativelyhigh melting temperatures, compared to InSe-type materials, so heatapplied to the phase-change layer should not affect the CIS or CIGSlayer. Finally, CIS materials tend to have good diode junctioncharacteristics.

The diode configuration 200 shown in FIG. 4 may be used as a detectiondevice in various data storage detection devices, such as photovoltaicdevices, cathodovoltaic devices, photoluminescent devices andcathodoluminescent devices, all described above.

FIG. 5 discloses a method of fabrication 300 of the diode structureshown in FIG. 4. Fabrication of the diode structure 200 starts at step310 with the selection of a substrate layer 210 from silicon or varioussilicon oxide materials. In the present embodiment, substrate 210 ispreferably of a material that can be doped with sodium, which promotesthe growth of the subsequent layers. Such material includes, but is notlimited to, silicon, silicon oxide, soda lime glass, and othersemiconductor materials. At step 312, an electrical potential fieldlayer, or back contact, 208 is fabricated on the surface of substrate210. Field layer 208 is fabricated from molybdenum (Mo) using asputtering technique, known to those skilled in the art, to a thicknessof 0.5–2.0 microns, with 1 micron being used in this embodiment. Othersuitable conductive materials are indium, titanium nitride, platinum,gold, zinc oxide, indium tin oxide and/or nickel.

Next, at step 314, second diode layer 204 is fabricated on field layer208. Other suitable phase-change materials may be used. In thisembodiment, diode layer 204 is preferably fabricated with a CIS compoundand is doped with a p-type dopant. Second diode layer 204 has athickness range of about 1 to 6 microns with 3.5 microns being used inthis embodiment, and is applied by elemental evaporation in a vacuum.Other methods of fabrication can include sputtering or electrodepositionusing an electrolyte of K₂SO₄, CuSO₄, In₂S(SO₄)₃, and/or SeO₂. Aspreviously mentioned, second diode layer 204 may also be doped withgallium to form CuInGaSe₂ or other CIGS compounds.

At step 316, after application of the second diode layer 204, the firstdiode layer 202 is fabricated on layer 208. In the present embodiment,first diode layer 202 is fabricated with In₂Se₃, being a natural n-typematerial. First layer 202 has a thickness range of 50–200 nm, with 100nm being used in this embodiment. First layer 202 may be applied byelemental evaporation in vacuum, which may be the same vacuum used toapply the second layer 204. In an alternative embodiment, fabrication offirst layer 202 can be performed with electrodeposition using anelectrolyte.

In an embodiment in which layer 204 is a CIGS material, layer 204 may begrown using a three-stage co-evaporation process. First, an (InGa)₂Se₃layer is deposited at 400° C., followed by evaporation of Cu and Se at550° C. to make the film slightly Cu-rich. The composition is restoredto slightly Cu-poor by the addition of (InGa)₂Se₃ again. Next, the InSelayer is thermally evaporated from elemental sources onto a heatedsurface. InSe is a natural n-type material, so no doping is required.Further, Cu can diffuse from the second diode layer of CuInSe into theInSe top layer to enhance the n-type doping.

At step 318, electric grid contacts 214 are fabricated on the surface oflayer 202 because the first layer of InSe 202 has a low electricalconductivity. Contacts 214 are made of a conductive material the same asor similar to field layer 208. For example, grid contacts 214 can bemade of Mo at a thickness ranging from 0.5 to 5 microns, with 1 micronused in this embodiment. Alternately, the grids may be made of othersuitable materials, such as Au or In. The material may besputter-deposited or made by any other suitable fabrication techniquesknown to those skilled in the art. Grid contacts 214 and field layer 208may be the connection points for applying a voltage source circuit tothe diode 200 during a read operation.

Testing has been performed on sample diodes as embodied in the presentinvention. The tests have included measuring the I–V characteristics ofthe diodes, performing quantum efficiency (optical), and performingelectron beam induced current (EBIC) measurements, which simulate a“read” mode for data storage.

FIG. 6 contains bar graphs showing EBIC gain as a function of emitterbeam voltage for diodes having In₂Se₃ as the top layer with In or Aucontacts and Si or CIGS as a bottom layer. The EBIC gain is diodecurrent output divided by electron beam current. In the first twosamples, In₂Se₃ was applied after the substrate was exposed to air. Thesecond two samples (in situ) were fabricated by depositing both layersin a vacuum without exposure to air during the process.

As shown in the FIG. 6 graph, the EBIC variations are not linear withvariations in emitter beam energy, because more energetic electron beamsresult in both more electron-hole pairs and deeper penetration. Although5 kV beams may be used, beams of 1 kV or less may be utilized in mostapplications. Beam current may vary from 0.5 nA to as high as 200 nA,with about 20 nA being used in some applications. As seen from thegraph, EBIC gain is considerably better with CIGS rather than Si as abottom diode layer. Also, the gain is substantially higher if the diodeis fabricated in a vacuum.

FIG. 7 shows the current-voltage characteristics for three samples ofthe first diode listed in FIG. 6. As shown in FIG. 7, the appliedvoltage causes essentially zero current until it reaches about 200 mV,after which there is a sharp increase in current. The open circuitvoltage is 5 mV and the short circuit current density is about 2 to 3 mAcm².

It is to be understood that the above-referenced arrangements areillustrative of the application for the principles of the presentinvention. Numerous modifications and alternative arrangements can bedevised without departing from the spirit and scope of the presentinvention while the present invention has been shown in the drawings anddescribed above in connection with the exemplary embodiments(s) of theinvention. It will be apparent to those of ordinary skill in the artthat numerous modifications can be made without departing from theprinciples and concepts of the invention as set forth in the claims.

1. A method of forming a diode structure for a phase-change data storagearray, having multiple thin film layers adapted to form a plurality ofdata storage cell diodes, comprising: depositing a first diode layer ofCIS material on a substrate; and depositing a second diode layer ofphase-change material on the first diode layer.
 2. The method of claim1, wherein the phase-change material is InSe material.
 3. The method ofclaim 1, wherein the phase-change material is In_(x)Se_(1−x) compound.4. The method of claim 1, wherein the steps of depositing the firstdiode layer and the second diode layer are done in a same vacuum system.5. The method of claim 1, wherein the steps of depositing the firstdiode layer and the second diode layer are done by electrodeposition. 6.The method of claim 1, wherein the steps of depositing the first diodelayer and the second diode layer are done using an electrolyte.
 7. Themethod of claim 1, further comprising depositing a field layer on thesubstrate prior to depositing the first diode layer.
 8. The method ofclaim 7, wherein the field layer is deposited by sputtering.
 9. Themethod of claim 7, wherein the step of depositing a field layer furthercomprises fabricating the field layer from a material selected from thegroup of conductive materials comprising molybdenum, indium, titaniumnitride, platinum, gold, zinc oxide, indium tin oxide and nickel. 10.The method of claim 1, wherein the steps of depositing the first diodelayer and the second diode layer are done by performing elementalevaporation in vacuum.
 11. The method of claim 1, wherein the steps ofdepositing the first diode layer and the second diode layer are done bysputtering.
 12. The method of claim 1, wherein the CIS material isp-doped.
 13. The method of claim 1, further comprising the step ofproviding a substrate selected from the group consisting of silicon,silicon oxide and soda lime glass.
 14. The method of claim 1, furthercomprising depositing contacts on a surface of the second diode layer.15. The method of claim 14 wherein the step of depositing contacts onthe surface of the second diode layer further comprises fabricatingcontacts from a material selected from the group of conductive materialsconsisting of indium and gold.
 16. A method of forming a diode structurefor a phase-change data storage array, having multiple thin film layersadapted to form a plurality of data storage cell diodes, comprising:providing a substrate; fabricating a field layer on the substrate;depositing a first diode layer of CIS material on the field layer; anddepositing a second diode layer of phase change material on the firstdiode layer.
 17. The method of claim 16, wherein the step of providing asubstrate comprises selecting a substrate material chosen from the groupof materials consisting of silicon, silicon oxide, and soda lime glass.18. The method of claim 16, wherein the step of fabricating a fieldlayer on the substrate comprises fabricating the field layer from amaterial selected from the group of conductive materials comprisingmolybdenum, indium, titanium nitride, platinum, gold, zinc oxide, indiumtin oxide and nickel.
 19. The method of claim 16, wherein the step ofdepositing the first diode layer and the second diode layer are done ina same vacuum system.
 20. The method of claim 16, wherein the step ofdepositing the first diode layer comprises forming the first diode layerfrom p-doped CIS material.
 21. The method of claim 16, wherein the stepof depositing the second diode layer comprises forming the second diodelayer from an InSe compound.
 22. A method of forming a diode structurefor a phase-change data storage array, having multiple thin film layersadapted to form a plurality of data storage cell diodes, comprising:providing a substrate; fabricating a field layer on the substrate;depositing a first diode layer of CIGS material on the field layer; anddepositing a second diode layer of phase change material on the firstdiode layer.